Spermatozoa migrate rapidly into a diluent brought into contact with semen. When this process was used to collect sperm in a diluent contained in Pasteur pipettes that were dipped into semen for 1 hour, concentrations of spermatozoa ranging from 0.6 to 5.7 X 10(6) cells/ml were obtained. This method yielded sperm suspensions free of seminal plasma and populations of spermatozoa that had increased motility ratings compared with the original semen. In contrast, removal of the seminal plasma by a procedure involving three dilutions and two gentle centrifugations resulted in a decrease in the average motility ratings. Determinations were made of the electrolyte, energy substrate, and trace element content of preovulatory human tubal fluid. A sperm diluent having an electrolyte composition similar to that of tubal fluid was formulated and used for collecting spermatozoa. Population of spermatozoa that migrated from semen into this diluent had significantly greater mean velocities than sperm in the semen. Moreover, the diluent proved to be a favourable medium for significantly delaying the deterioration of sperm progression over a 24-hour period.